1. Field of the Invention
This invention relates to the field of electrical signal control, and more specifically to binary logic gates for magnetic bubble circuits.
2. Description of the Prior Art
Technology related to storage and propagation of magnetic bubbles has existed for a number of years and is well documented in the prior art. The term "magnetic bubbles" refers to small domains of reversed magnetization which exist in certain uniaxial magnetic thin films when a bias field of appropriate magnitude is applied. Storage and propagation of the bubbles is possible by arranging magnetically permeable thin film elements on a substrate to form a path for the bubbles, which propagate along the path in response to a rotating field. Proven advantages of magnetic bubble circuits incude high bit packing density, low power consumption, low cost, and high reliability.
While bubble storage and propagation technology is rather well developed, the use of this technology to create effective binary logic gates has been less successful. A logic gate must provide alternative paths for bubble propagation such that bubble interaction determines which of the alternate paths is taken. Typically a propagation path will branch into "hard" and "easy" paths, with a bubble traversing the "easy" path in the absence of any interaction. The difficulty with such gate designs is that the "hard" path tends to collapse bubbles, and thus establishes an upper limit for the bias field. At the path junction, bubbles tend to strip-out between the alternate paths, thereby establishing a lower limit for the bias field. Strip-out refers to an elongation of a bubble across several adjacent path elements, and is conventionally regarded as a device failure. The device operating margin is the difference between the upper and lower operating limits on the bias field, and for prior art bubble logic gates this margin is undesirably small.
Another difficulty of prior art logic devices arises from the presence of a strong interaction force between the bubbles and individual propagation patterns, which tends to overpower the bubble-to-bubble interaction force. Bubbles thus must be brought very close together in order to obtain logic action; the close spacing leads to bubble collapse at high bias, and bubble strip-out failure at low bias. Disclosures illustrating logic gates with provision for close bubble interaction include U.S. Pat. Nos. 4,117,543 to Minnick and 3,723,716 to Bobeck et al. Both teach wide columnar structures of chevrons, but their close chevron spacing makes the gates so susceptible to strip-out that they cannot be operated in a normal bias range.
In order to reduce the force exerted on a bubble by the propagation paths, a logic gate can utilize thick spacing between the bubble material and the permalloy propagation patterns. This causes the permissible propagation speeds to be substantially reduced, and the use of circuits containing passive bubble replicators is rendered impractical.
An alternative scheme for bubble logic is disclosed in U.S. Pat. No. 4,200,924 to Ishihara et al., which includes chevron columns wherein the vertical distance between adjacent chevrons is gradually increased along the propagation path and the chevron width is gradually decreased. Strip-out is allowed to occur in those cases where only a single bubble is provided into the circuit, and the strip is subsequently converted back into a bubble on a predetermined path by a taffy-pulling arrangement. That is, as the strip becomes excessively enlongated by the continuously widening chevron gaps, it eventually breaks off from the narrow chevrons and consolidates on the wider center element. This approach requires an abnormally high bias field to aid in detaching the ends of the strip. This high bias tends to collapse bubbles on the narrow chevrons when two bubbles are input simultaneously. Further, propagation speed in this device is undesirably limited due to the large vertical spacing of the chevron elements which provide only a low density of magnetic poles along the length of an elongated strip domain.